1. Field of Invention
This invention relates to an absolute position optical displacement-sensing device that senses absolute positions in two dimensions.
2. Description of Related Art
Absolute position encoders use a scale structure that allows the position of a readhead relative to a scale to be determined without requiring any reference to a given starting point along the scale. A variety of 1-dimensional (1D) absolute position encoders are known that use multiple information patterns extending in parallel along a 1-dimensional scale. Also, 1D and 2-dimensional (2D) codes have been used as a means to identify positions along ID scales. However, the structures of such absolute position encoders that are fundamentally 1-dimensional are generally not adaptable or combinable to provide a high resolution and high accuracy absolute position measurement at an arbitrary position in a two-dimensional plane.
A two-dimensional (2D) incremental (non-absolute) position encoder providing high resolution and high accuracy at an arbitrary position in a 2D plane is disclose in U.S. Pat. No. 5,104,225 to Masreliez. However, the grating scale and readhead disclosed in the 225 patent are not adaptable to provide absolute position measurement.
Numerous 2D bar code systems are known. However, the xe2x80x9cinformation storagexe2x80x9d structures of such 2D bar code systems are generally not well suited to act as a 2D scale for high-resolution absolute position determinations. Furthermore, it is not clear how to array such codes into a continuous 2D scale and reliably discriminate between adjacent codes.
A 2D absolute scale pattern for a position measuring system is a two-dimensional pattern carried on a two-dimensional surface of a scale member. The inventors of this invention have determined that ID absolute position measurement systems and 2D grating incremental measurement systems are generally difficult, costly, or impossible to adapt for use as high-resolution 2D absolute measurement systems. Furthermore, the patterns of various 2D bar code configurations are generally not well suited to act as patterns for 2D scales for high-resolution absolute position determination.
Such bar code patterns are especially poorly suited for providing high resolution position measurements at high speed. It should be appreciated that determining the position of complex and variable pattern structures to a high resolution generally requires comparing such structures to a similarly complex and variable template or detector pattern, or the like. Such comparisons are generally too time consuming for tracking the motion of such patterns relative to a detector with both the high resolution and high speed generally desired for position measurement and motion control applications. Furthermore, when multiple complex patterns are arrayed adjacent to each other in a 2D pattern to form a continuous 2D scale, discrimination between the various individual patterns adds further signal processing complexity and further increases the difficulty of determining the position of such patterns with both high resolution and high speed. Moreover, generating such patterns over an extended two-dimensional area that are unique within that area while at the same time aligning these patterns with an underlying continuous periodic grid in order to provide an accurate and high-resolution absolute measurement scale is technically difficult and/or costly.
Attempting to circumvent the foregoing problems by reducing the spatial resolution and/or the distribution of such patterns will generally degrade the ability to determine the positions of these patterns with high resolution, which generally depends on the spatial frequencies or xe2x80x9cinformation densityxe2x80x9d, that is, the xe2x80x9cdensity of transitionsxe2x80x9d present in a position measurement scale. Alternatively, attempting to circumvent the foregoing problems by reducing the complexity of the patterns will general reduce the amount area that can be everywhere uniquely identified. That is, the potential range of the 2D absolute scale will be reduced.
An optical absolute position encoder that could avoid any one or more of these disadvantages would be useful. It should be appreciated that, in general, it is the particular arrangement of a 2D absolute scale pattern that provides a 2D absolute position measuring system that provides high speed position measurement over a long range with high resolution and accuracy and at a reasonable cost.
This invention provides 2D optical absolute position encoders that have relatively small readhead sizes.
This invention separately provides a 2D scale usable with optical absolute position encoders that has a 2D integrated scale that includes both a code portion and a periodic portion.
This invention further provides 2D optical absolute position encoders having a 2D scale pattern that integrates both periodic and code portions in advantageous proportions.
This invention separately provides 2D optical absolute position encoders that have both long scale lengths along each of the two dimensions and high resolution.
This invention further provides 2D optical absolute position encoders that obtain long scale lengths and high resolution by using a 2D integrated scale that includes a code portion that allows for long scale lengths along each of the two dimensions and a periodic portion that allows for high-resolution determinations of the relative position between the readhead and the scale.
This invention further provides a 2D scale that has a 2D integrated scale where the code portion and the periodic portion occur alternately along each of the two dimensions of the 2D scale.
This invention additionally provides a 2D scale where the code portions and the periodic portions that occur alternately along each of the two dimensions of the scale are adjacent to one another in both of the two dimensions.
This invention separately provides methods for determining the absolute position of a readhead of an optical absolute position encoder in two dimensions relative to a 2D scale of the optical absolute position encoder from an image of a 2D integrated scale that includes both non-periodic code portions and periodic portions.
This invention further provides methods for determining the absolute position of the readhead relative to the 2D scale along each dimension by locating a code portion appearing within a 2D image of the 2D scale and determining a first-resolution 2D position along each dimension based on the code appearing in the located code portion.
This invention additionally provides methods for determining the absolute position of the readhead relative to the 2D scale along each dimension by locating a predetermined portion appearing within the 2D image of the 2D scale and determining a second-resolution position along each dimension based on the 2D location of the predetermined portion within the 2D image of the 2D scale.
This invention also provides methods for determining the absolute position of the readhead relative to the 2D scale along each dimension by determining an offset distance along each dimension between a periodic portion appearing within the 2D image and a periodic reference structure and determining a third-resolution position along each dimension based on at least one of the offset distances.
In various exemplary embodiments of the scale according to this invention, the scale includes a 2D integrated scale pattern that extends in two dimensions. The integrated scale pattern includes one or more periodic portions that extend in each of the two dimensions, where each periodic portion defines a plurality of periodic elements that have extents along each of the two dimensions. The integrated scale pattern also includes non-periodic code portions, which are distributed in two dimensions within and/or between the one or more periodic portions. Each non-periodic code portion includes a unique set or group of code elements and thus identifies a specific 2D location within the scale. That is, each unique set or group of code elements defines a first location along a first one of the two dimensions and a second location along a second one of the two dimensions.
In various exemplary embodiments, each non-periodic code portion includes a predetermined portion that has an extent along each of the two dimensions. In each dimension, the predetermined portion may be a single element or space having predetermined characteristics, or may be a predetermined pattern of elements. This predetermined portion allows the readhead signals arising from the code portions of the scale to be rapidly located and/or distinguished from the readhead signals arising from other portions of the scale.
In various other exemplary embodiments according to this invention, the periodic scale elements are placed along the scale in each dimension to coincide with an underlying incremental pitch for that dimension that is continuous and spatially synchronized along that dimension between the periodic portions, even though, in some exemplary embodiments, the continuity of the periodic elements along that dimension may be interrupted in some areas of the scale.
In various other exemplary embodiments according to this invention, for at least one of the two dimensions, the extents of at least some of the individual code elements of the non-periodic code portions are narrower along that dimension than the extents of the individual periodic scale elements.
In various other exemplary embodiments according to this invention, for at least one of the two dimensions, one set of the code elements may be spaced apart along that dimension from an adjacent set of code elements up to a determined limit based on the extent of a detector array of the readhead along that dimension and a magnification along that dimension applied to the scale image by the readhead.
In various exemplary embodiments, each set of code elements in effect indicates the position or measurement value along each dimension of a local datum feature to provide a coarsely-spaced 2D absolute position value. The local datum feature is associated with a plurality of periodic scale elements for each dimension. In various other exemplary embodiments according to this invention, a detector array of the readhead further determines the position along each of the two dimensions of the local datum feature relative to the detector array to a higher resolution that is finer than the period along each dimension of the periodic scale elements. In various other exemplary embodiments according to this invention, the detector array of the readhead further determines the position along each dimension of at least some of the periodic scale elements relative to the detector array and/or the readhead to provide the highest resolution 2D absolute position measurement. In various other exemplary embodiments according to this invention, the detector array pitch is chosen such that, along at least one of the two dimensions, there are a plurality of detector elements for each periodic scale element imaged by the array. In various exemplary embodiments, the detector array pitch is chosen such that, along at least one of the two dimensions, there are a plurality of detector elements for each code element imaged by the array.
In various exemplary embodiments of the 2D absolute scale according to this invention, the sets of code elements form a 2D sequence of code words extending across the 2D scale. In various other exemplary embodiments according to this invention, the sequence of code words directly indicates the corresponding 2D position within the scale. In various other exemplary embodiments according to this invention, the code words are converted to absolute position measurement values along each of the two dimensions using a decoder look-up table.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.